Activation of PKC affects the ventricular restitution properties and arrhythmogenesis through L-type Ca+ current

OBJECTIVE

To investigate the role of protein kinase C (PKC) in action potential duration (APD) restitution and ventricular tachyarrhythmias (VAs).

METHODS AND RESULTS

Rabbits hearts were isolated and prepared for Langendorff perfusion technique. The stimuli-extra-stimulus (S1-S2) method and dynamic S1 pacing protocol were performed to construct APD restitution and to induce APD alternans or VA, respectively, at 10 sites throughout the ventricular chamber. Administration of phorbol-12-myristate-13-acetate (PMA) (100 nM) (n = 15) greatly steepened the restitution curves (Smax > 1) (p < .01) at each site compared to the control group (n = 15). Furthermore, treatment with PMA also induced larger spatial dispersions of Smax (p < .05) and decreased the thresholds of the VA and APD alternans (p < .01). However, perfused with the PKC inhibitor, bisindolylmaleimide (BIM) (500 nM) (n = 10), reversibly flattened the APD restitution curves at each site (Smax < 1), decreased the spatial dispersions of Smax, and increased the thresholds of APD alternans and VA. According to the results of patch-clamp, peak amplitude of L-type Ca2+ current was significantly increased by addition of PMA compared with control (CTL) group (p < .05). Antagonize this current with verapamil (n = 10) can fully inhibited the PMA induced increasing of Smax and inducibility of VA and alternans.

CONCLUSION

PKC activation increased the dispersion of APD restitution and thus led to occurrence of VA, which possibly related to the increased Ca2+ influx.

The autonomic nervous system in atrial fibrillation-pathophysiology and non-invasive assessment

The autonomic nervous system plays a crucial role in atrial fibrillation pathophysiology. Parasympathetic hyperactivity result in a shortening of the action potential duration, a reduction of the conduction wavelength, and as such facilitates reentry in the presence of triggers. Further, autonomic remodeling of atrial myocytes in AF includes progressive sympathetic hyperinnervation by increased atrial sympathetic nerve density and sympathetic atrial nerve sprouting. Knowledge on the pathophysiological process in AF, including the contribution of the autonomic nervous system, may in the near future guide personalized AF management. This review focuses on the role of the autonomic nervous system in atrial fibrillation pathophysiology and non-invasive assessment of the autonomic nervous system.

Advances in basic and translational research in atrial fibrillation

Atrial fibrillation (AF) is a very common cardiac arrhythmia with an estimated prevalence of 33.5 million patients globally. It is associated with an increased risk of death, stroke and peripheral embolism. Although genetic studies have identified a growing number of genes associated with AF, the definitive impact of these genetic findings is yet to be established. Several mechanisms, including electrical, structural and neural remodelling of atrial tissue, have been proposed to contribute to the development of AF. Despite over a century of exploration, the molecular and cellular mechanisms underlying AF have not been fully established. Current antiarrhythmic drugs are associated with a significant rate of adverse events and management of AF using ablation is not optimal, especially in cases of persistent AF. This review discusses recent advances in our understanding and management of AF, including new concepts of epidemiology, genetics and pathophysiological mechanisms. We review the current status of antiarrhythmic drug therapy for AF, new potential agents, as well as mechanism-based AF ablation. This article is part of the theme issue 'The heartbeat: its molecular basis and physiological mechanisms'.

Transient outward K+ current can strongly modulate action potential duration and initiate alternans in the human atrium

Efforts to identify the mechanisms for the initiation and maintenance of human atrial fibrillation (AF) often focus on changes in specific elements of the atrial "substrate," i.e., its electrophysiological properties and/or structural components. We used experimentally validated mathematical models of the human atrial myocyte action potential (AP), both at baseline in sinus rhythm (SR) and in the setting of chronic AF, to identify significant contributions of the Ca2+-independent transient outward K+ current ( Ito) to electrophysiological instability and arrhythmia initiation. First, we explored whether changes in the recovery or restitution of the AP duration (APD) and/or its dynamic stability (alternans) can be modulated by Ito. Recent reports have identified disease-dependent spatial differences in expression levels of the specific K+ channel α-subunits that underlie Ito in the left atrium. Therefore, we studied the functional consequences of this by deletion of 50% of native Ito (Kv4.3) and its replacement with Kv1.4. Interestingly, significant changes in the short-term stability of the human atrial AP waveform were revealed. Specifically, this K+ channel isoform switch produced discontinuities in the initial slope of the APD restitution curve and appearance of APD alternans. This pattern of in silico results resembles some of the changes observed in high-resolution clinical electrophysiological recordings. Important insights into mechanisms for these changes emerged from known biophysical properties (reactivation kinetics) of Kv1.4 versus those of Kv4.3. These results suggest new approaches for pharmacological management of AF, based on molecular properties of specific K+ isoforms and their changed expression during progressive disease. NEW & NOTEWORTHY Clinical studies identify oscillations (alternans) in action potential (AP) duration as a predictor for atrial fibrillation (AF). The abbreviated AP in AF also involves changes in K+ currents and early repolarization of the AP. Our simulations illustrate how substitution of Kv1.4 for the native current, Kv4.3, alters the AP waveform and enhances alternans. Knowledge of this "isoform switch" and related dynamics in the AF substrate may guide new approaches for detection and management of AF.

Pro-arrhythmic atrial phenotypes in incrementally paced murine Pgc1β-/- hearts: effects of age

New Findings

What is the central question of this study?

Can we experimentally replicate atrial pro‐arrhythmic phenotypes associated with important chronic clinical conditions, including physical inactivity, obesity, diabetes mellitus and metabolic syndrome, compromising mitochondrial function, and clarify their electrophysiological basis?

What is the main finding and its importance?

Electrocardiographic and intracellular cardiomyocyte recording at progressively incremented pacing rates demonstrated age‐dependent atrial arrhythmic phenotypes in Langendorff‐perfused murine Pgc1β^−/− hearts for the first time. We attributed these to compromised action potential conduction and excitation wavefronts, whilst excluding alterations in recovery properties or temporal electrophysiological instabilities, clarifying these pro‐arrhythmic changes in chronic metabolic disease.

Atrial arrhythmias, most commonly manifesting as atrial fibrillation, represent a major clinical problem. The incidence of atrial fibrillation increases with both age and conditions associated with energetic dysfunction. Atrial arrhythmic phenotypes were compared in young (12–16 week) and aged (>52 week) wild‐type (WT) and peroxisome proliferative activated receptor, gamma, coactivator 1 beta (Ppargc1b)‐deficient (Pgc1β^−/−) Langendorff‐perfused hearts, previously used to model mitochondrial energetic disorder. Electrophysiological explorations were performed using simultaneous whole‐heart ECG and intracellular atrial action potential (AP) recordings. Two stimulation protocols were used: an S1S2 protocol, which imposed extrasystolic stimuli at successively decremented intervals following regular pulse trains; and a regular pacing protocol at successively incremented frequencies. Aged Pgc1β^−/− hearts showed greater atrial arrhythmogenicity, presenting as atrial tachycardia and ectopic activity. Maximal rates of AP depolarization (dV/dt_max) were reduced in Pgc1β^−/− hearts. Action potential latencies were increased by the Pgc1β^−/− genotype, with an added interactive effect of age. In contrast, AP durations to 90% recovery (APD₉₀) were shorter in Pgc1β^−/− hearts despite similar atrial effective recovery periods amongst the different groups. These findings accompanied paradoxical decreases in the incidence and duration of alternans in the aged and Pgc1β^−/− hearts. Limiting slopes of restitution curves of APD₉₀ against diastolic interval were correspondingly reduced interactively by Pgc1β^−/− genotype and age. In contrast, reduced AP wavelengths were associated with Pgc1β^−/− genotype, both independently and interacting with age, through the basic cycle lengths explored, with the aged Pgc1β^−/− hearts showing the shortest wavelengths. These findings thus implicate AP wavelength in possible mechanisms for the atrial arrhythmic changes reported here.

Alternans in atria: Mechanisms and clinical relevance

Atrial fibrillation is the most common sustained arrhythmia and its prevalence is rapidly rising with the aging of the population. Cardiac alternans, defined as cyclic beat-to-beat alternations in contraction force, action potential (AP) duration and intracellular Ca²⁺ release at constant stimulation rate, has been associated with the development of ventricular arrhythmias. Recent clinical data also provide strong evidence that alternans plays a central role in arrhythmogenesis in atria. The aim of this article is to review the mechanisms that are responsible for repolarization alternans and contribute to the transition from spatially concordant alternans to the more arrhythmogenic spatially discordant alternans in atria.

Effects of Dantrolene Treatment on Ventricular Electrophysiology and Arrhythmogenesis in Rats With Chronic β-Adrenergic Receptor Activation

Dantrolene, which is primarily used to treat malignant hyperthermia, has recently been suggested for the prevention of arrhythmogenesis in various animal models. In this study, the effects of dantrolene treatment on electrophysiological properties and ventricular arrhythmias (VAs) in rats with chronic β-adrenergic receptor (β-AR) activation were investigated. Rats were randomized to treatment with saline (control group), isoproterenol (ISO; ISO group), or ISO + dantrolene (ID group) for 2 weeks. An electrophysiological study was performed to assess action potential duration restitution (APDR) and induce action potential duration (APD) alternans or VA in vitro. The protein levels of Cav1.2, sarcoplasmic reticulum Ca2+-ATPase (SERCA2a), and ryanodine receptor 2 (RyR2) were detected by Western blot. Compared with the control group, chronic administration of ISO significantly increased APD, the maximum slope (Smax) of APDR curve, and the spatial dispersions of Smax and APD (all P < .01), and all effects were attenuated by dantrolene treatment (all P < .05). Additionally, chronic ISO administration significantly reduced the protein levels of SERCA2 and RyR2, but increased the Cav1.2 protein expression (all P < .05). However, compared with the ISO group, dantrolene treatment preserved SERCA2a and RyR2 protein levels and decreased Cav1.2 protein levels in the ID group (all P < .05). The intracellular Ca2+ ([Ca2+]i) levels measured by incubating isolated cardiomyocytes with Fluo-3/alveolar macrophages were significantly increased in the ISO group compared with the control group ( P < .01). Dantrolene treatment markedly reduced the rise of [Ca2+]i levels caused by chronic administration of ISO ( P < .05). Dantrolene treatment also prevented the reductions in the APD alternans and VA thresholds induced by chronic ISO stimulation (all P < .05). These data suggest that dantrolene stabilizes ventricular electrophysiological characteristics and increases the expression of key sarcoplasmic reticulum calcium cycling proteins to reduce vulnerability to VA in rats with chronic β-AR activation.

Renal sympathetic denervation modulates ventricular electrophysiology and has a protective effect on ischaemia-induced ventricular arrhythmia

Recently, a beneficial effect of renal sympathetic denervation (RSD) has been seen in patients with ventricular electrical storm. However, the effect of RSD on ventricular electrophysiology remains unclear. Thirty-three mongrel dogs were included in the present study. Renal sympathetic denervation was performed by radiofrequency ablation of the adventitial surface of the renal artery. In group 1 (n = 8), programmed stimulation was performed before and after RSD to determine the ventricular effective refractory period (ERP) and action potential duration (APD) restitution properties. The same parameters were measured in five other animals that underwent sham RSD to serve as controls. In group 2 (n = 10), acute myocardial ischaemia (AMI) was induced by ligating the proximal left anterior descending coronary artery after the performance of RSD, and the incidence of ventricular arrhythmia (VA) was calculated during 1 h of recording. In another 10 dogs (group 3), AMI was induced and VA was measured with sham RSD. In group 1, RSD significantly prolonged ventricular ERP and APD, reduced the maximal slope (Smax) of the restitution curve and suppressed APD alternans at each site. Renal sympathetic denervation also significantly decreased the spatial dispersion of ERP, APD and Smax. In the five control animals, no significant electrophysiological change was detected after sham RSD. The occurrence of spontaneous VA during 1 h of AMI in group 2 was significantly lower than that in group 3. These data suggest that RSD stabilizes ventricular electrophysiological properties in normal hearts and reduces the occurrence of VA in hearts experiencing AMI.

The role of action potential alternans in the initiation of atrial fibrillation in humans: a review and future directions

This review highlights the role of atrial monophasic action potential duration (APD) in understanding atrial electrical properties in paroxysmal, persistent, and permanent atrial fibrillation (AF) states. Alternans of APD and rate maladaptation in a spatially divergent way appear mechanistically involved in AF initiation, development, and persistence. The underlying pathophysiology warrants further investigation.

Effects of ganglionated plexi ablation on ventricular electrophysiological properties in normal hearts and after acute myocardial ischemia

BACKGROUND

Ganglionated plexi (GP) ablation has been shown to play an important role in atrial fibrillation (AF) initiation and maintenance. Also, GP ablation increases chances for prevention of AF recurrence. This study investigated the effects of GP ablation on ventricular electrophysiological properties in normal dog hearts and after acute myocardial ischemia (AMI).

METHODS

Fifty anesthetized dogs were assigned into normal heart group (n=16) and AMI heart group (n=34). Ventricular dynamic restitution, effective refractory period (ERP), electrical alternans and ventricular fibrillation threshold (VFT) were measured before and after GP ablation in the normal heart group. In the AMI heart group, the incidence of ventricular arrhythmias and VFT were determined.

RESULTS

In the normal heart group, GP ablation significantly prolonged ERP, facilitated electrical alternans but did not increase ERP dispersion, the slope of restitution curves and its spatial dispersion. Also, GP ablation did not cause significant change of VFT. In the AMI heart group, the incidence of ventricular arrhythmias after GP ablation was significantly higher than that in the control group or the GP plus stellate ganglion (SG) ablation group (P<0.05). Spontaneous VF occurred in 8/12, 1/10 and 2/12 dogs in the GP ablation group, the GP plus SG ablation group and the control group, respectively (P<0.05). VFT in the GP ablation group showed a decreased trend though a significant difference was not achieved compared with the control or the GP plus SG ablation group.

CONCLUSIONS

GP ablation increases the risk of ventricular arrhythmias in the AMI heart compared to the normal heart.

Mechanisms of human atrial fibrillation initiation: clinical and computational studies of repolarization restitution and activation latency

BACKGROUND

Mechanisms of atrial fibrillation (AF) initiation are incompletely understood. We hypothesized that rate-dependent changes (restitution) in action potential duration (APD) and activation latency are central targets for clinical interventions that induce AF. We tested this hypothesis using clinical experiments and computer models.

METHODS AND RESULTS

In 50 patients (20 persistent, 23 paroxysmal AF, 7 controls), we used monophasic action potential catheters to define left atrial APD restitution, activation latency, and AF incidence from premature extrastimuli. Isoproterenol (n=14), adenosine (n=10), or rapid pacing (n=36) was then initiated to determine impact on these parameters. Compared with baseline in AF patients, isoproterenol and rapid pacing decreased activation latency (64±14 versus 31±13 versus 24±14 ms; P<0.05), steepened maximum APD restitution slope (0.8±0.7 versus 1.7±0.5 versus 1.1±0.5; P<0.05), and increased AF incidence (12% versus 64% versus 84%; P<0.05). Conversely, adenosine shortened APD (P<0.05), yet increased activation latency (86±27 ms; P=0.002) so that maximum APD restitution slope did not steepen (1.0±0.5; P=NS), and AF incidence was unchanged (10%; P=NS). In controls, no intervention steepened APD restitution or initiated AF. Computational modeling revealed that isoproterenol steepened APD restitution by increased L-type calcium current and decreased activation latency via enhanced rapid delayed potassium reactifier current inactivation, whereas rapid pacing steepened APD restitution via increased cardiac inward potassium rectifier current.

CONCLUSIONS

Steep APD restitution is a common pathway for AF initiation by isoproterenol and tachycardia via reduced activation latency that enables engagement of steep APD restitution at rapid rates. Modeling suggests that AF initiation from each intervention uses distinct ionic mechanisms. This insight may help design interventions to prevent AF.